Fuel injectors useful for applications such as gas turbine combustion engines, direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also function to prepare the fuel for mixing with air prior to combustion. Each injector typically has an inlet fitting connected either directly or via tubing to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chambers. A fuel passage (e.g., a tube or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle. The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustion chamber. Additional concentric and/or series combustion chambers each require their own arrangements of nozzles that can be supported separately or on common stems. The fuel provided by the injectors is mixed with air and ignited, so that the expanding gases of combustion can, for example, move rapidly across and rotate turbine blades in a gas turbine engine to power an aircraft, or in other appropriate manners in other combustion applications.
One technique for creating an efficient spray of fuel is to provide the air in a swirling motion surrounding the fuel spray. The swirling air causes the fuel to have a swirling component of motion, which causes quick and uniform mixing of the fuel with the air, and thereby better atomization. A well-atomized spray of fuel results in lower flame temperatures in the combustor, which more efficiently and completely burns the fuel and results in lower emissions of pollutants such as Nitrous Oxide (NOx). It is therefore desirable to maximize the swirling flow of air around the fuel spray to maximize the efficiency of the engine.
Because of limited fuel pressure availability and a wide range of required fuel flow, many fuel injectors include pilot and secondary nozzles, with only the pilot nozzles used during start-up, and both the pilot and secondary nozzles used during higher power operation. There is no flow or only low flow through the secondary nozzles during start-up, idle and lower power operation. Such injectors can be more efficient and cleaner-burning than single nozzle fuel injectors, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement. The pilot and secondary nozzles can be contained within the same nozzle stem assembly or can be supported in separate nozzle assemblies.
One particularly useful spray nozzle is shown and described in Simmons, et al., U.S. Pat. No. 5,435,884, which is owned by the assignee of the present invention. In the Simmons patent, a spray nozzle is formed from multiple plates using chemical etching. A bowl shaped swirl chamber, a spray orifice, non-radial feed slot and an annular feed annulus are each formed in one or more of the plates. Such a spray nozzle has been found to be efficient in its performance, provide low emissions, and be relatively easy to manufacture compared with many mechanically-formed nozzles, particularly for nozzles with a low Flow Number (the relation of the rate of liquid flow output to the applied inlet pressure) and small dimensions.
One particularly useful application of the Simmons spray nozzle has been developed by the assignee of the present invention. The application includes supporting an air swirler assembly downstream of the spray nozzle to impart a swirling component of motion to the fuel. The air swirler assembly is also formed from multiple plates, and includes a cylindrical air swirler passage in at least one of the plates, located in co-axial relation to the spray orifice of the nozzle assembly such that fuel directed through the spray orifice passes through the air swirler passage and swirling air can be imparted to the fuel. A pair of air feed slots are in fluid communication with each air swirler passage and extend in non-radial relation thereto for supplying air to be swirled in the air swirler passage. This application is described in more detail in U.S. patent application Ser. No. 09/794,490, filed Feb. 27, 2001, for “Integrated Fluid Injection and Mixing System”.
One issue that occurs with injectors having pilot and secondary nozzles is that the nozzles are typically supported close to each other, and the air flow for the secondary nozzles can interfere with, and sometimes even quench, the flame from the pilot nozzles, particularly when the fuel flow from the secondary nozzles is low or off (start-up, low power or idle conditions). This is because the air flow for the secondary nozzles is typically not controlled, and is always present regardless of whether fuel is being supplied to the secondary nozzles. This is particularly an issue with applications which require multiple nozzles to be supported in a small space, where interaction between the sprays can cause a significant reduction in the efficiency of the engine. Reducing the air flow is one solution, but in doing so it becomes more difficult to achieve uniform and rapid mixing of the air and fuel and hence the atomization suffers. As discussed above, a poorly-atomized fuel spray causes higher flame temperatures during combustion, resulting in higher emissions of Nitrous Oxide.
With current trends toward developing even more efficient and cleaner-burning combustors, which require increased air flow for even more efficient operation, it is a continuing challenge to develop improved fuel injector assemblies to properly deliver fuel to a combustion chamber for operation of the gas turbine engine, which fits into a small envelope, and can be manufactured and assembled in an economical manner.